The research proposed here will evaluate competing hypotheses concerning age-related constraints on the acquisition of a second language (L2). According to the maturational state hypothesis, which assumes the existence of a critical period, ultimate performance in an L2 becomes poorer with increasing age because the ability to learn an l2 diminishes. According to the interference hypothesis, on the other hand, success in the L2 varies as a function of the potency of a competing native language (L1) system. This hypothesis assumes that the L1 and L2 systems cannot be isolated completely from one another. Two projects are to be undertaken. Project 1 will examine adult Korean/English (KE) bilinguals who arrived in the United States between the ages of 2-24 years and have lived in the United States for at least 8 years. The bilinguals' performance in both English (the L2) and Korean (L1) will be compared to that of adult monolingual speakers of English and Korean Project 2 will examine adult and child KE bilinguals who differ in age (7-10, 12-15, or 21-24 years) and experience in English (1 or 4 years of residence in the United States). The performance of these bilinguals in both English and Korean will be assessed longitudinally by comparing their performance to that of age-matched groups of monolinguals at three times over a 12-month period. In both projects, acquisition of English morphosyntax will be assessed using a Grammaticality Judgment Test, and acquisition of English phonology will be assessed through listening tests. (native English listeners will rate English sentences for degree in foreign accent. More detailed analyses will examine the production of particular vowels and consonants.) Effects of learning English phonology on Korean phonology will be assessed using similar procedures. The interference hypothesists, but not the maturational state hypothesis, predicts that phonological accuracy in English and Korean will be inversely related. Statistical analyses will be undertaken to determine if variations in the age at which the Korean Ss began to learn English has a similar effect on their rate of acquisition and ultimate attainment in English phonology and morphosyntax. Their responses to an 84-item questionnaire will be submitted to a principal components analysis. The underlying factors it yields will be submitted to multiple regression analyses examining overall degree of foreign accent and d' scores derived from the Grammaticality Judgment Test. The question of interest here is whether the same subject factors (e.g., degree of motivation to learn English, the L 1/L2 usage ratio) accounts for performance in the two linguistic domains. Taken together, the results should enable the investigators to determine if a critical period(s) that is independent of the influence of L1 structures exists for the acquisition of L2 phonology and morphosyntax.